multi: add ChanEdgePolicyFromWire constructor for ChannelEdgePolicy

Add ChanEdgePolicyFromWire to construct ChannelEdgePolicy from channel
update messages, centralizing v1/v2 field mapping.
Update call sites to use the helper:

- discovery/gossiper: handleChanUpdate
- graph/builder: ApplyChannelUpdate
- routing/router_test: ApplyChannelUpdate test helper

This consolidates update-to-policy conversion logic across versions.

graph/db/models/channel_edge_policy.go

@@ -99,6 +99,56 @@ type ChannelEdgePolicy struct {
 	ExtraSignedFields map[uint64][]byte
 }
 
+// ChanEdgePolicyFromWire constructs a ChannelEdgePolicy from a channel update
+// message.
+func ChanEdgePolicyFromWire(scid uint64,
+	update lnwire.ChannelUpdate) (*ChannelEdgePolicy, error) {
+
+	switch upd := update.(type) {
+	case *lnwire.ChannelUpdate1:
+		//nolint:ll
+		return &ChannelEdgePolicy{
+			Version:                   lnwire.GossipVersion1,
+			SigBytes:                  upd.Signature.ToSignatureBytes(),
+			ChannelID:                 scid,
+			LastUpdate:                time.Unix(int64(upd.Timestamp), 0),
+			MessageFlags:              upd.MessageFlags,
+			ChannelFlags:              upd.ChannelFlags,
+			TimeLockDelta:             upd.TimeLockDelta,
+			MinHTLC:                   upd.HtlcMinimumMsat,
+			MaxHTLC:                   upd.HtlcMaximumMsat,
+			FeeBaseMSat:               lnwire.MilliSatoshi(upd.BaseFee),
+			FeeProportionalMillionths: lnwire.MilliSatoshi(upd.FeeRate),
+			InboundFee:                upd.InboundFee.ValOpt(),
+			ExtraOpaqueData:           upd.ExtraOpaqueData,
+		}, nil
+
+	case *lnwire.ChannelUpdate2:
+		return &ChannelEdgePolicy{
+			Version:         lnwire.GossipVersion2,
+			SigBytes:        upd.Signature.Val.ToSignatureBytes(),
+			ChannelID:       scid,
+			LastBlockHeight: upd.BlockHeight.Val,
+			SecondPeer:      upd.SecondPeer.IsSome(),
+			DisableFlags:    upd.DisabledFlags.Val,
+			TimeLockDelta:   upd.CLTVExpiryDelta.Val,
+			MinHTLC:         upd.HTLCMinimumMsat.Val,
+			MaxHTLC:         upd.HTLCMaximumMsat.Val,
+			FeeBaseMSat: lnwire.MilliSatoshi(
+				upd.FeeBaseMsat.Val,
+			),
+			FeeProportionalMillionths: lnwire.MilliSatoshi(
+				upd.FeeProportionalMillionths.Val,
+			),
+			InboundFee:        upd.InboundFee.ValOpt(),
+			ExtraSignedFields: upd.ExtraSignedFields,
+		}, nil
+	}
+
+	return nil, fmt.Errorf("unknown channel update version: %v",
+		update.MsgType())
+}
+
 // IsNode1 returns true if this policy was announced by the channel's node_1.
 func (c *ChannelEdgePolicy) IsNode1() bool {
 	if c.Version == lnwire.GossipVersion1 {